OTTAWA — In what’s billed as a landmark study, an international team of experts — including a researcher from the University of Ottawa — has produced the most accurate assessment of ice sheet losses in Antarctica and Greenland to date.

Their findings, to be published Friday in the journal Science, confirm that Antarctica and Greenland are both losing ice. The melting has generated a 11.1-millimetre increase in global sea levels since 1992 — one-fifth of the total sea level rise over that period.

Moreover, the rate of sea level rise is accelerating, rising by nearly one millimetre per year now compared with 0.27 millimetres per year in the 1990s, the researchers say.

Melting ice sheets now account for about one-third of the annual rise in sea levels. Melting mountain glaciers and the thermal expansion of the ocean account for the rest.

According to the study, the change has been particularly acute in Greenland, where the rate of ice loss has increased almost five-fold since the mid-1990s. Overall, Greenland and Antarctica are now losing more than three times as much ice as they were in the 1990s, the study found.

“With this new data, we can say with confidence that the ice sheets have been losing mass over the last 10 to 15 years,” said the University of Ottawa’s Glenn Milne, the only Canadian participant in the study.

Since 1998, there have been at least 29 estimates of ice sheet losses based on various satellite techniques. However, even though they were often looking at the same satellite findings, researchers could not agree on how to interpret the data.

The last major assessment in 2007 by the International Panel on Climate Change was so broad it was impossible to say whether the Antarctic ice sheets were growing or shrinking.

In the new study, 47 researchers from 26 laboratories around the world reconciled the differences by combining measurements collected by different types of satellites and matching time periods and survey areas. That produced estimates believed to be more than twice as accurate as those in the 2007 IPCC report.

The researchers estimate that between 1992 and 2011, the Greenland ice sheet lost 2,940 gigatonnes of ice and ice sheets in Antarctica lost 1,320 gigatonnes. (One gigatonne equals one billion tonnes.)

Results in the Antarctic varied by region. The Western Antarctic and Antarctic Peninsula both lost ice during the study period, but ice sheets in the Eastern Antarctic — which occupies more than three-quarters of the continent — actually grew in size during the final years of the survey.

Milne said the ice losses are significant because of their impact on sea levels. “When you talk about millimetres per year, people don’t really understand how much ice that actually is,” he said. About 360 gigatonnes of ice must melt to raise worldwide sea levels by one millimetre, he said.

The geological record indicates that ice sheet losses generated annual sea level rises in excess of 10 millimetres a year during past periods of climatic change, the study says.

“The prospect of such changes in the future are of greatest concern. Even the modest rises in ocean temperatures that are predicted over the coming century could trigger significant ice sheet mass loss through enhanced melting of ice shelves and outlet glaciers.”

According to the 2006 Stern Review on the Economics of Climate Change, 200 million people live in coastal flood plains and assets worth $1 trillion lie within a metre of the current sea level.

Extrapolating the study’s findings into the future is an uncertain exercise, Milne acknowledged. “This is what we call a non-linear system. You can’t just get a straight line today and push it into the future. Chances are you’re going to depart from a straight line.”

Even so, he said, most experts agree that a simple extrapolation of current trends “would give you a minimum estimate of what to expect in 100 years.” That would yield a 30-centimetre rise in sea levels, he said. “I think we’re already committed to at least that amount.”

Predicting the upper limit is more difficult, partly because ice sheets can change very quickly, Milne said. But if ice sheet melting continues to accelerate, sea levels could rise by as much as a metre by the end of the century, he said.

One caveat is that the researchers’ study covers a relatively short time frame in climate change terms, Milne said. That means the acceleration in Greenland could be just a blip.

But Milne said that’s unlikely, pointing out that this past summer, for the first time in centuries, the whole surface of the Greenland ice sheet was melting.

“All of these pieces of evidence are coming together to suggest that what Greenland’s doing right now is quite anomalous,” he said. “Chances are it’s not just a blip that we’ll recover from.”